Activity results in Android Kotlin - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of ActivityResultLauncher in Android Kotlin?

It is used to launch another activity and receive a result back asynchronously, replacing the older startActivityForResult method.

beginner

How do you register for an activity result in Android Kotlin?

You use registerForActivityResult() with an ActivityResultContract and a callback to handle the result.

intermediate

What is an ActivityResultContract?

It defines the input type needed to start an activity and the output type returned as a result, helping to standardize communication between activities.

intermediate

Why is the new activity result API preferred over startActivityForResult?

Because it is lifecycle-aware, safer, and easier to manage results without overriding onActivityResult.

beginner

What happens if you try to launch an activity result without registering the launcher first?

The app will crash or the result will not be delivered because the launcher is not initialized to handle the result callback.

Which method replaces startActivityForResult in modern Android Kotlin development?

A launchActivity

B startActivity

C onActivityResult

D registerForActivityResult

registerForActivityResult is the new recommended way to launch activities for results.

What is the role of the callback passed to registerForActivityResult?

A To handle the result returned from the launched activity

B To start the activity

C To register the activity in the manifest

D To create a new activity instance

The callback receives the result data when the launched activity finishes.

Which class defines the input and output types for activity results?

A Bundle

B Intent

C ActivityResultContract

D ActivityResultLauncher

ActivityResultContract specifies how to create the intent and parse the result.

What lifecycle benefit does the new activity result API provide?

A It is lifecycle-aware and avoids memory leaks

B It runs on a background thread

C It automatically restarts the activity

D It disables lifecycle callbacks

The API respects the lifecycle of the component to prevent leaks and crashes.

How do you launch the activity after registering the launcher?

A Call <code>startActivityForResult()</code>

B Call <code>launch()</code> on the <code>ActivityResultLauncher</code>

C Call <code>onActivityResult()</code>

D Call <code>registerForActivityResult()</code> again

You use the launch() method on the registered launcher to start the activity.

Explain how to use the new activity result API in Android Kotlin to get data back from another activity.

Think about how you prepare, start, and receive results from another screen.

Describe the advantages of the new activity result API compared to the old startActivityForResult method.

Focus on safety and ease of use improvements.

Practice

1. What is the main purpose of registerForActivityResult in Android Kotlin?

easy

A. To create a new fragment instance

B. To start a new activity without expecting a result

C. To register a broadcast receiver

D. To safely receive data back from another activity

Solution

1. Step 1: Understand the role of registerForActivityResult

   This function is designed to handle results from activities started for a result safely and simply.

2. Step 2: Compare with other options

   Starting an activity without expecting a result or registering broadcast receivers are unrelated to this function.

3. Final Answer:

   To safely receive data back from another activity -> Option D

4. Quick Check:

   Activity result handling = A [OK]

Hint: Remember: registerForActivityResult handles returned data safely [OK]

Common Mistakes:

• Confusing it with starting activities without results
• Thinking it registers broadcast receivers
• Assuming it creates fragments

2. Which of the following is the correct syntax to register an activity result launcher in Kotlin?

easy

A. val launcher = registerForActivityResult(ActivityResultContracts.StartActivityForResult()) { result -> /* handle result */ }

B. val launcher = registerActivityResult(ActivityResultContracts.StartActivityForResult()) { result -> /* handle result */ }

C. val launcher = registerForResult(ActivityResultContracts.StartActivityForResult()) { result -> /* handle result */ }

D. val launcher = registerActivity(ActivityResultContracts.StartActivityForResult()) { result -> /* handle result */ }

Solution

1. Step 1: Identify the correct function name

   The correct function is registerForActivityResult, not any shortened or altered form.

2. Step 2: Check the contract and lambda syntax

   The contract ActivityResultContracts.StartActivityForResult() and lambda parameter { result -> } are correctly used in val launcher = registerForActivityResult(ActivityResultContracts.StartActivityForResult()) { result -> /* handle result */ }.

3. Final Answer:

   val launcher = registerForActivityResult(ActivityResultContracts.StartActivityForResult()) { result -> /* handle result */ } -> Option A

4. Quick Check:

   Correct function name and syntax = B [OK]

Hint: Look for exact function name: registerForActivityResult [OK]

Common Mistakes:

• Using incorrect function names like registerActivityResult
• Omitting parentheses in contract instantiation
• Wrong lambda syntax

3. Given the following code snippet, what will be printed if the launched activity returns RESULT_OK with intent data containing key "username" = "Alice"?

val launcher = registerForActivityResult(ActivityResultContracts.StartActivityForResult()) { result ->
    if (result.resultCode == Activity.RESULT_OK) {
        val name = result.data?.getStringExtra("username")
        println(name)
    } else {
        println("No data")
    }
}

medium

A. Crash due to null pointer

B. Alice

C. null

D. No data

Solution

1. Step 1: Check the result code condition

   The code checks if result.resultCode == Activity.RESULT_OK, which is true in this case.

2. Step 2: Extract the "username" from intent data

   The intent data contains key "username" with value "Alice", so name will be "Alice" and printed.

3. Final Answer:

   Alice -> Option B

4. Quick Check:

   Result OK with data returns value = D [OK]

Hint: Check resultCode and safely unwrap intent data [OK]

Common Mistakes:

• Ignoring resultCode and printing 'No data'
• Assuming data is null without checking
• Not using safe call operator on intent data

4. Identify the error in this code snippet for handling activity results:

val launcher = registerForActivityResult(ActivityResultContracts.StartActivityForResult()) { result ->
    val name = result.data!!.getStringExtra("username")
    if (result.resultCode == Activity.RESULT_OK) {
        println(name)
    }
}

medium

A. Unsafe call on result.data before checking resultCode

B. Missing lambda parameter in registerForActivityResult

C. Incorrect contract used for activity result

D. No error, code is correct

Solution

1. Step 1: Analyze the order of operations

   The code force unwraps result.data!! before checking if result.resultCode == Activity.RESULT_OK.

2. Step 2: Understand potential crash

   If the result is not OK or data is null, force unwrapping causes a crash. The check should come before accessing data.

3. Final Answer:

   Unsafe call on result.data before checking resultCode -> Option A

4. Quick Check:

   Check resultCode before accessing data = A [OK]

Hint: Always check resultCode before using intent data [OK]

Common Mistakes:

• Force unwrapping nullable data without checks
• Accessing data before verifying resultCode
• Ignoring null safety in Kotlin

5. You want to start an activity to pick a contact and get the contact's URI back. Which approach correctly uses registerForActivityResult to achieve this?

hard

A. val pickContact = registerForActivityResult(ActivityResultContracts.RequestPermission()) { granted -> if (granted) println("Permission granted") }

B. val pickContact = registerForActivityResult(ActivityResultContracts.StartActivityForResult()) { result -> println(result.data?.getData().toString()) }

C. val pickContact = registerForActivityResult(ActivityResultContracts.PickContact()) { uri -> if (uri != null) println(uri.toString()) }

D. val pickContact = registerForActivityResult(ActivityResultContracts.TakePicture()) { success -> if (success) println("Picture taken") }

Solution

1. Step 1: Identify the correct contract for picking a contact

   ActivityResultContracts.PickContact() is designed to pick a contact and return its URI.

2. Step 2: Check the lambda parameter and usage

   The lambda receives a URI nullable parameter, which is safely checked before printing.

3. Step 3: Eliminate other options

   val pickContact = registerForActivityResult(ActivityResultContracts.StartActivityForResult()) { result -> println(result.data?.getData().toString()) } uses a generic contract but does not check resultCode; A and D are unrelated contracts for permissions and taking pictures.

4. Final Answer:

   val pickContact = registerForActivityResult(ActivityResultContracts.PickContact()) { uri -> if (uri != null) println(uri.toString()) } -> Option C

5. Quick Check:

   Use PickContact contract for contact URI = C [OK]

Hint: Use specific contracts like PickContact for contact picking [OK]

Common Mistakes:

• Using StartActivityForResult without checking resultCode
• Confusing permission or picture contracts with contact picking
• Not handling nullable URI safely